The function to be invoked when an error occurs on this Media object.
Note that onError should be initialized before setting the source of the media
to ensure that the errors can be handled appropriately.

Defines the String which specifies the URI of the media;
It must be an absolute URI, such as "file:///media.fxm".

Defines the String which specifies the URI of the media;
It must be an absolute URI, such as "file:///media.fxm".
If it is relative to the codebase the __DIR__ may
be used, as in "{__DIR__}/media.fxm".
Supported protocols include "file:", "http:", and "jar:".
Currently, only audio in MP3, AU, and WAV containers are supported
from within jar files.

the Object for the specided key.
The class of this is often a
String
but could be another class
depending on the media type and the key value.
Note that metadata may not be available for all media types.